Reportable Segments - Revenue information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information
Net investment income	$ 832	$ 880	$ 1,662	$ 2,470
Realized capital gains (losses)	440	197	682	1,183
Total revenues	11,500	11,069	32,773	33,203
Property-Liability
Segment Reporting Information
Premiums Earned, Net	8,952	8,782	26,696	25,970
Net investment income	422	448	802	1,210
Realized capital gains (losses)	292	163	571	916
Total revenues	9,858	9,588	28,624	28,657
Property-Liability | Auto
Segment Reporting Information
Premiums Earned, Net	6,210	6,080	18,537	18,045
Property-Liability | Homeowners
Segment Reporting Information
Premiums Earned, Net	2,073	1,997	6,164	5,890
Property-Liability | Other personal lines
Segment Reporting Information
Premiums Earned, Net	486	469	1,435	1,390
Property-Liability | Commercial lines
Segment Reporting Information
Premiums Earned, Net	183	236	560	645
Allstate Protection
Segment Reporting Information
Premiums Earned, Net	8,952	8,782	26,696	25,970
Discontinued Lines and Coverages
Segment Reporting Information
Premiums Earned, Net	0	0	0	0
Service Businesses
Segment Reporting Information
Intersegment premiums and service fees	36	44	109	110
Net investment income	12	11	33	30
Realized capital gains (losses)	14	4	9	21
Total revenues	498	418	1,404	1,215
Service Businesses | Consumer product protection plans
Segment Reporting Information
Insurance premiums, commissions and fees	236	163	661	461
Service Businesses | Roadside assistance
Segment Reporting Information
Insurance premiums, commissions and fees	48	57	143	183
Service Businesses | Finance and insurance products
Segment Reporting Information
Insurance premiums, commissions and fees	100	92	294	268
Allstate Life
Segment Reporting Information
Net investment income	123	128	374	380
Realized capital gains (losses)	9	5	(3)	1
Total revenues	490	495	1,457	1,473
Allstate Life | Traditional life insurance premiums
Segment Reporting Information
Insurance premiums, commissions and fees	154	155	471	465
Allstate Life | Accident and health insurance premiums
Segment Reporting Information
Insurance premiums, commissions and fees	0	0	1	1
Allstate Life | Interest-sensitive life insurance contract charges
Segment Reporting Information
Insurance premiums, commissions and fees	176	176	530	535
Allstate Benefits
Segment Reporting Information
Net investment income	18	21	58	61
Realized capital gains (losses)	3	2	0	8
Total revenues	308	314	890	932
Allstate Benefits | Traditional life insurance premiums
Segment Reporting Information
Insurance premiums, commissions and fees	12	12	33	31
Allstate Benefits | Accident and health insurance premiums
Segment Reporting Information
Insurance premiums, commissions and fees	235	250	704	746
Allstate Benefits | Interest-sensitive life insurance contract charges
Segment Reporting Information
Insurance premiums, commissions and fees	40	29	95	86
Allstate Annuities
Segment Reporting Information
Net investment income	245	251	358	737
Realized capital gains (losses)	112	20	88	224
Total revenues	360	274	453	971
Allstate Annuities | Fixed annuities contract charges
Segment Reporting Information
Insurance premiums, commissions and fees	3	3	7	10
Operating Segments | Property-Liability
Segment Reporting Information
Insurance premiums, commissions and fees	192	195	555	561
Operating Segments | Service Businesses
Segment Reporting Information
Insurance premiums, commissions and fees	52	47	155	142
Operating Segments | Allstate Life
Segment Reporting Information
Insurance premiums, commissions and fees	28	31	84	91
Operating Segments | Corporate and Other
Segment Reporting Information
Net investment income	12	21	37	52
Realized capital gains (losses)	10	3	17	13
Total revenues	22	24	54	65
Intersegment Eliminations
Segment Reporting Information
Total revenues	$ (36)	$ (44)	$ (109)	$ (110)